Glass mats have been used as separators in Pb-acid storage batteries for many years. One particular such battery is the so-called "gas recombinant" battery. In a gas-recombinant battery (i.e., single or multicell) oxygen generated at one plate moves across the interelectrode gap to the opposite polarity plate and recombines with hydrogen generated thereat. The interelectrode gap is filled with a bibulous, fibrous glass mat which immobilizes the electrolyte and prevents dendrite growth between adjacent plates. One class of separators for such batteries is described in McClelland et al U.S. Pat. No. 3,862,861 and comprises a fibrous glass mat which is unsaturated with electrolyte and has fiber diameters between 0.2 and about 10 microns and a surface area between about 0.1 and 20 square meters/gram of silica.
Multicell bipolar batteries are also well known in the art. They may either be of the conventional or gas-recombinant type and employ either one of two types of bipolar electrodes. The first, or "face-to-face" type of bipolar electrode, utilizes an electrolyte impervious, conductive sheet having a first polarity, electrochemically active material applied (e.g., pasted) onto one face of the sheet and an opposite polarity, electrochemically active material applied to the opposite face of the sheet. These electrodes are stacked in the battery such that the opposite polarity faces of adjacent electrodes oppose each other across an electrolyte-filled gap, and are separated one from the other by an electrolyte-permeable separator. The second, or "side-by-side", type of bipolar electrode for a multicell battery comprises an electrically conductive substrate (e.g., a grid work of conductive wires) having two separate, substantially coplanar, side-by-side electrode portions (hereafter plates) thereon each of which contains an electrochemically active material of opposite polarity to the other. The two opposite polarity plates are electrically connected to each other by an electrically conductive link which comprises a central segment of the shared conductive substrate which is free of electrochemically active material and lies intermediate the two opposite polarity plates and in essentially the same plane as the plates. Such side-by-side bipolar electrodes, and a multicell battery made therefrom, are shown in Schilke et al U.S. Pat. No. 3,167,456, assigned to the assignee of the present invention. Schilke et al's, side-by-side bipolar electrodes are arranged in overlapping fashion so as to form a plurality of cell elements each housed in a separate cell compartment of a container and comprising a stack of the positive and negative polarity plates of different bipolar electrodes alternately interleaved one with the other. The bipolar electrodes are arranged such that the first polarity plate of each bipolar electrode resides in one compartment and the opposite polarity plate of the same bipolar electrode resides in an adjacent cell compartment in the fashion depicted in FIG. 1 hereof. Conductive links extending between adjacent compartments electrically link the opposite polarity plates together and form the battery's intercell connectors. A more modern version of such a battery is shown in Bish et al U.S. Pat. No. 5,106,708, assigned to the assignee of the present invention.
Assembling gas-recombinant batteries (i.e., conventional or bipolar) is a problem because the glass mats are very resilient and are typically compressed about 25% to about 30% during assembly of the cell element. The cell element must be held in this compressed state during and after assembly into the container. Typically compressive pressures of about 8 psi to about 12 psi are required to compress such a stack depending on the exact density of the mat and degree of compression. Of course, the stack exerts a similar back pressure or expansive force as the stack seeks to return to an unstressed (i.e., uncompressed) state. This problem is even more acute when assembling multicell, bipolar batteries of the type described in Bish et al U.S. Pat. No. 5,106,708 where this force tending to expand the element is exerted upon the cover. One technique that has been proposed for such cells is to stack the plates and separators loosely in their cell compartments, lay the battery cover atop the stack, and apply pressure to the cover to compress the stacks and hold them compressed until the cover is secured to the container. This approach has disadvantages however. In this regard, containers and covers are preferably made from thermoplastic materials which are heat-sealed together by locally heating both pieces, pressing them together and allowing the joint to cool and bond the container and cover together. When the cover is used to hold the glass mats compressed during heat sealing, compressive force must be maintained on the cover throughout the cooling stage else the spring-like expansive force of the compressed glass tends to push the cover off of the container while the heat-sealed joint is till hot and soft.
It is the principal object of the present invention to provide a battery and method of assembling wherein compressed fibrous mat separators are compressed and maintained undercompression by a discrete retainer which prevents the expansion forces of the cell element stack from dislocating the battery cover from the battery container. This and other objects and advantages will become more readily apparent from the detailed description thereof which follows.